JESS: [EXTERNAL] No cast needed when inspecting shadow facts' data members?

[Henrique Lopes Cardoso]

Hi,

I've just noticed an interesting behavior of Jess.
I was working with a couple of classes like this:

public class X {
Object obj;
// getter and setter for obj
...
}

public class Y {
int i;
// getter and setter for i
...
}

Then in Jess I wrote:

(deftemplate X (declare (from-class X)))
(deftemplate a (slot s))

(defrule r
(X (obj ?o))
(test (eq ((?o getClass) getSimpleName) Y))
(a (s ?o.i))
=
(printout t ?o.i crlf))

(bind ?x (new X))
(bind ?y (new Y))
(?y setI 123)
(?x setObj ?y)
(add ?x)
(run)

This actually works! My surprise is related with the fact that the obj data
member is declared as an Object, and the _i_ data member only exists for
instances of Y. Despite this, the rule is able to get ?o.i in both the LHS
adn the RHS (no cast needed). Of course if I remove the test in the rule
and add to working memory an instance of X for which the obj is not an Y, I
get a runtime exception.

Is there anything I should know about the appropriateness of
implementations such as this? Best practices?

Thank you in advance.

Henrique

RE: JESS: [EXTERNAL] No cast needed when inspecting shadow facts' data members?

[Friedman-Hill, Ernest]

Jess doesn't actually try to look for the I member until the code actually 
runs, so it really has no choice but to accept the code as written. This really 
isn't any different from how other dynamically typed languages behave; Java, 
being a strongly/statically typed language that would not allow this kind of 
code is actually unusual these days. Ruby, Python, Scala, Groovy, etc would all 
allow this sort of thing, no casting needed.

From: owner-jess-us...@sandia.gov [mailto:owner-jess-us...@sandia.gov] On 
Behalf Of Henrique Lopes Cardoso
Sent: Friday, May 10, 2013 9:16 AM
To: jess-users
Subject: JESS: [EXTERNAL] No cast needed when inspecting shadow facts' data 
members?

Hi,

I've just noticed an interesting behavior of Jess.
I was working with a couple of classes like this:

public class X {
Object obj;
// getter and setter for obj
...
}

public class Y {
int i;
// getter and setter for i
...
}

Then in Jess I wrote:

(deftemplate X (declare (from-class X)))
(deftemplate a (slot s))

(defrule r
(X (obj ?o))
(test (eq ((?o getClass) getSimpleName) Y))
(a (s ?o.i))
=
(printout t ?o.i crlf))

(bind ?x (new X))
(bind ?y (new Y))
(?y setI 123)
(?x setObj ?y)
(add ?x)
(run)

This actually works! My surprise is related with the fact that the obj data 
member is declared as an Object, and the _i_ data member only exists for 
instances of Y. Despite this, the rule is able to get ?o.i in both the LHS adn 
the RHS (no cast needed). Of course if I remove the test in the rule and add to 
working memory an instance of X for which the obj is not an Y, I get a runtime 
exception.

Is there anything I should know about the appropriateness of implementations 
such as this? Best practices?

Thank you in advance.

Henrique